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Interior insulated cornice for use under low-heel roof trusses?

user-198087 | Posted in Energy Efficiency and Durability on

Has anyone found, used, or developed an interior insulated air-seal cornice for the wall to ceiling connection below a “low heel” truss eave condition that does not accommodate full attic insulation thickness?

The interior cornice in this condition would be a substitute for a spray-foam wedge in the framing where the truss bears on the wall. The wall is existing 1973 2×4 construction with a double top plate, so the wall insulation terminates at the underside of the top plate. How much of a condensation issue would it be to insulate with an interior cornice, lowering the winter temperature of the wall’s top plate?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Has anyone found, used, or developed an interior insulated air-seal cornice for the wall to ceiling connection below a "low heel" truss eave condition that does not accommodate full attic insulation thickness?"

    A. Yes. This solution was investigated by two researchers from the University of Illinois at Urbana-Champaign, Jeffrey Gordon and William Rose. They invented, produced, and tested a prototype that they called "EPS crown molding." I wrote an article about their research which was published in the October 2008 issue of Energy Design Update. A photo of their EPS crown molding is shown below.

    Excerpts from the article:

    "To investigate whether additional insulation would lower the chance of mold development in cold ceiling corners, Gordon and Rose conducted a study at 18 single-family homes in a Chippewa community in Belcourt, North Dakota. The ranch-style homes were all owned by a public housing agency, the Turtle Mountain Housing Authority (TMHA). According to Gordon and Rose, the TMHA homes had “numerous cases of houses exhibiting moisture problems at the wall/ceiling juncture.” Each of the retrofit options (the insulation pack method, the top plate pillow method, and the crown molding method) was installed on the north side of five houses. ...

    "All of the retrofit work was completed in early September 2004. From December 2004 through March 2005, the researchers monitored and collected data from a variety of equipment, including outdoor weather stations, indoor temperature and humidity data loggers, and thermocouple temperature sensors.

    "The performance of the retrofit work was assessed by a variety of methods, including computer modeling, thermocouple data review, and infrared photo analysis. Gordon and Rose reached two major conclusions:
    - Because of thermal bridging, the temperature of the ceiling directly under each roof truss is more critical than the temperature of the ceiling at the center of each framing bay.
    - None of the three insulation retrofit measures made much of a difference.

    "The roof trusses in the North Dakota houses created cold stripes on the ceilings. Gordon and Rose reported, “In addressing the moisture problem at the wall/ceiling juncture, the coldest and most critical temperature is at the truss. Retrofit insulation treatments cannot significantly change the temperature at the truss.”

    "Elsewhere, Gordon and Rose explained, “All three treatments provided little to no improvement at the truss locations. There was some indication that the exterior treatments made a slight improvement. The findings for the interior crown were more ambiguous. Ultimately, the thermal bridging that results in the temperature depression at the truss was not remediated by any of the insulation techniques.” ....

    "Interviewed by phone, Rose readily admitted that their insulation strategies didn’t work. “These methods had so little effect that they aren’t worth considering,” Rose told EDU. The research findings sent Rose back to the drawing board, leading him to suggest a new remedy: wrapping the entire exterior (walls and roof) of the home with rigid foam.

    "“Reducing the thermal bridge effect at a truss doesn’t lend itself very well to makeshift efforts,” Rose explained. “Any strategy that allows a truss member to be continuous from the inside to the outside means that the existing conditions can’t be substantially improved. To do it right, you have to cut off the overhangs, insulate the exterior of the wall and roof with rigid foam, and then reattach the overhangs.”

    "Rose’s suggested remedy is similar to the “chainsaw retrofit” method pioneered by Rob Dumont in the 1980s."


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